In the field of cables, such as LAN cables, certain common insulators are used for forming the twisted pair insulation as well as the outer jacket. Common polymers used include FEP (Fluorinated Ethylene Propylene) and PE (Polyethylene). Although these insulations provide good flame resistant properties needed to meet fire safety standards, such as UL riser and UL plenum ratings, they have relatively high dielectric constants, tending to cause insertion loss in the signals propagated along the cables.
One approach in the prior art for reducing the dielectric constant of an insulator is to introduce air or gas into the polymer insulation during the extrusion process in order to foam the insulation. Typically, chemical or physical foaming of the insulation (dielectric) is used to provide material reduction and an improvement to the transmission properties for data communication cables. However, there are several limitations with the foaming process.
Physical foaming of the dielectric typically includes injecting an inert gas such as nitrogen or carbon dioxide into a molten polymer under heat and pressure while inside an extruder. The gases are injected in the extruder in a low pressure area of the screw and absorbed by the molten polymer. The gas passes through the extruder, while dissolved in the molten polymer, until the polymer exits the extruder. Once the captivated gas inside the polymer is exposed to atmospheric pressures, it combines at a nucleation point and forms bubbles within the insulation. This process requires additional equipment such as a gas pressurization unit to inject the gas at a critical velocity into the polymer and complex screw designs such as multi-stage screws and an extrusion barrel with gas injection ports.
Chemical foaming is also used to create bubbles within the dielectric without the need for additional equipment. However, chemical foaming is not used as frequently as physical foaming because this method also has negative drawbacks inherent in the process. Chemical foaming is done by mixing a number of additives, at a given ratio, with the main polymer. Typically, a “nucleating agent” such as Boron Nitride is added to the main polymer to provide the point at which gas bubbles are formed and grow. The nucleating agent is distributed into the polymer with or without the use of mixing elements that are located on the extrusion screw. Increasing the amount of sites available within the polymer allows for more locations for bubbles to start. Additionally, another chemical is blended into the polymer to generate the gas. These additives, known as a “blowing agents” are mixed with the nucleating agent at the same time. The blowing agent may have a melting point much lower that the main polymer, so that once the material reaches a given temperature it degrades and produces a gas (vapor) within the melt. The vapor from the degraded material forms a bubble at the closest nucleation site. Chemical foam and gas injection extrusion lines are difficult to control and run slowly with low yields.
Another approach to reducing the dielectric constant in a conductor is to simply create cavities in the insulation surrounding the conductors. However, prior art attempts in this area are unsatisfactory, particularly with respect to insulation for each individual conductor in a twisted pair. For example, U.S. Pat. No. 5,922,155 shows an insulation provided for coaxial cables. Here, the insulator is extruded resulting in a wheel shaped insulator surrounding the central conductor of the coaxial cable. However, such a technique is not equally applicable to placing an insulator in an individual conductor from a twisted pair which is significantly smaller in diameter. Further disadvantages of the '155 patent methodology include the fact that the extrusion die used is a complicated multi-component die, requiring significant upkeep. Also, there is no ability to adjust the pressure within the cavities during extrusion without shutdown and re-tooling.
Thus, the prior art does not exhibit any means for both reducing the dielectric constant of the insulation, such as insulation on the individual copper conductors of a twisted pair communication cable, without the costly addition of materials need to foam the insulation.